Hyperbolic exciton polaritons in a van der Waals magnet

Francesco L Ruta; Shuai Zhang; Yinming Shao; Samuel L Moore; Swagata Acharya; Zhiyuan Sun; Siyuan Qiu; Johannes Geurs; Brian S Y Kim; Matthew Fu; Daniel G Chica; Dimitar Pashov; Xiaodong Xu; Di Xiao; Milan Delor; X-Y Zhu; Andrew J Millis; Xavier Roy; James C Hone; Cory R Dean; Mikhail I Katsnelson; Mark van Schilfgaarde; D N Basov

doi:10.1038/s41467-023-44100-6

Hyperbolic exciton polaritons in a van der Waals magnet

Nat Commun. 2023 Dec 13;14(1):8261. doi: 10.1038/s41467-023-44100-6.

Authors

Francesco L Ruta^#^{1

2}, Shuai Zhang^#³, Yinming Shao^#⁴, Samuel L Moore⁴, Swagata Acharya⁵, Zhiyuan Sun⁴, Siyuan Qiu⁴, Johannes Geurs^{4

6}, Brian S Y Kim^{4

7}, Matthew Fu⁴, Daniel G Chica⁸, Dimitar Pashov⁹, Xiaodong Xu^{10

11}, Di Xiao^{10

11}, Milan Delor⁸, X-Y Zhu⁸, Andrew J Millis^{4

12}, Xavier Roy⁸, James C Hone⁷, Cory R Dean⁴, Mikhail I Katsnelson¹³, Mark van Schilfgaarde⁵, D N Basov¹⁴

Affiliations

¹ Department of Physics, Columbia University, New York, NY, USA. f.ruta@columbia.edu.
² Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA. f.ruta@columbia.edu.
³ Department of Physics, Columbia University, New York, NY, USA. sz2822@columbia.edu.
⁴ Department of Physics, Columbia University, New York, NY, USA.
⁵ National Renewable Energy Laboratory, Golden, CO, USA.
⁶ Columbia Nano Initiative, Columbia University, New York, NY, USA.
⁷ Department of Mechanical Engineering, Columbia University, New York, NY, USA.
⁸ Department of Chemistry, Columbia University, New York, NY, USA.
⁹ Theory and Simulation of Condensed Matter, King's College London, London, UK.
¹⁰ Department of Physics, University of Washington, Seattle, WA, USA.
¹¹ Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA.
¹² Center for Computational Quantum Physics, Flatiron Institute, New York, NY, USA.
¹³ Institute for Molecules and Materials, Radboud University, Nijmegen, Netherlands.
¹⁴ Department of Physics, Columbia University, New York, NY, USA. db3056@columbia.edu.

^# Contributed equally.

Abstract

Exciton polaritons are quasiparticles of photons coupled strongly to bound electron-hole pairs, manifesting as an anti-crossing light dispersion near an exciton resonance. Highly anisotropic semiconductors with opposite-signed permittivities along different crystal axes are predicted to host exotic modes inside the anti-crossing called hyperbolic exciton polaritons (HEPs), which confine light subdiffractionally with enhanced density of states. Here, we show observational evidence of steady-state HEPs in the van der Waals magnet chromium sulfide bromide (CrSBr) using a cryogenic near-infrared near-field microscope. At low temperatures, in the magnetically-ordered state, anisotropic exciton resonances sharpen, driving the permittivity negative along one crystal axis and enabling HEP propagation. We characterize HEP momentum and losses in CrSBr, also demonstrating coupling to excitonic sidebands and enhancement by magnetic order: which boosts exciton spectral weight via wavefunction delocalization. Our findings open new pathways to nanoscale manipulation of excitons and light, including routes to magnetic, nonlocal, and quantum polaritonics.

Abstract

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